Abstract: During May and June in 2012,Karenia mikimotoi blooms occurred in the coastal sea areas of Fujian Province,which were responsible for massive mortalities of abalones and huge economic losses.However,little is known about whether the mortalities of abalones were associated with the detrimental effects of K.mikimotoi or not.Under laboratory conditions,we investigated the effects of K.mikimotoi on the survival of the abalone Haliotis discus hannai at different environmental conditions.Meanwhile,we also monitored the changes of some main environmental factors in the conditions of aeration and non-aeration,including DO,pH,and total ammonia (TAN).At the aeration condition,DO was in the range of 7.3~7.8 mg/L,pH of 7.5~8.2,TAN of 0.0035~0.084 mg/L,which were all suitable for the survival of abalone.At this condition,the mortality rates reached 33.3% in 48 h,and reached 100% in 96 h when exposed to K.mikimotoi at the cell amounts of 3107/L,which indicated that the rapid mortalities of abalones were caused by the detrimental effects of K.mikimotoi. At the non-aeration condition,DO was in the range of 7.1~1.8 mg/L,pH of 7.2~8.1,TAN of 0.007~0.051 mg/L,the mortality rates reached 100% in 16h when exposed to K.mikimotoi at the cell amounts of 3107/L.The results showed that the low oxygen condition made the toxic effects stronger.Therein,we can speculated that the rapid and massive mortalities of abalone during the occurrence of K.mikimotoi bloom were related with the toxic effects of the dinoflagellate K.mikimotoi,furthermore,the low oxygen conditions during blooms may strengthen the effects.